1. [Technical Field to which the Invention Belongs]
The present invention relates to a method of producing xylitol, particularly to a process for producing a highly pure xylitol product by which xylitol can be obtained by separating it in high purity from a culture broth of a xylitol-producing microorganism (e.g., a xylitol enzymatic reaction broth or a xylitol fermentation broth) by ingeniously utilizing ion-exchange resins.
2. [Prior Art]
Xylitol is widely present also in nature, and is a naturally occurring sweetener contained also in fruits and vegetables such as strawberry, cauliflower, lettuce, and spinach. Xylitol has a property of a lower calorific value though it exhibits a sweetness level comparable to sucrose. In addition, it has an excellent property, i.e., an anti-dental caries property.
Xylitol is, as disclosed in U.S. Pat. No. 4,008,825, produced mainly by reduction of xylose which is, in turn, obtainable through hydrolysis of xylan contained in plant materials. In this method, since the content of xylan in plant materials is low, there exist problems that th e yield of xylose resulting from such hydrolysis is low and, in addition, that many impurities such as other pentoses and hexsoses accompany the xylose. Furthermore, generation of plenty of liquid wastes during the hydrolysis and much cost of treating the liquid wastes for conserving the environment are other problems.
Xylose thus obtained is then converted to xylitol through reduction in the presence of a catalyst. When the xylose is reduced, the above-mentioned impurities such as other pentoses and hexsoses accompanying the xylose are reduced concurrently with the xylose, and converted into sugar alcohols having a structure resembling xylitol. Therefore, as disclosed in Japanese Patent Application Laid-Open (Kokai) No. 2690/1980, Japanese Patent Publication (Kokoku) Nos. 15054/1986, and 35169/1983, and the like, it is necessary to separate the contaminant many kinds of sugar alcohols from the aimed-at xylitol.
Accordingly, it is desirable to separate xylose from a hydrolysate of xylan as purely as possible. Low purity xylose may complicate steps required for purification of xylitol obtained by reduction of xylose, and, as a result, lead to increase of the cost of producing xylitol.
For solving the problem, it is desired to develop a method for producing xylitol where the starting material is easily available, and wastes are only generated in a small amount as well as the cost is inexpensive.
As such a method, it is investigated to produce xylitol from easily available glucose using a microorganism, without intervening xylose as an intermediate. For example, Published Japanese Translation of PCT International Publication for Patent Application No. 505522/1996 discloses a method for producing xylitol from glucose, using a yeast.
A method for producing xylitol from glucose, using a microorganism, can eliminate relatively severe conditions and danger accompanying hydrolysis of xylan contained in plant materials and hydrogenation of xylose using a metal catalyst, while enabling to produce xylitol under mild conditions. As a result, it leads to reduction of the cost for production.
Xylitol formed by microorganisms, however, may be accompanied with organic acids (for example, citric acid, acetic acid, gluconic acid, fumaric acid and malic acid) as by-products, and sugars and sugar alcohols (for example, glucose, D-arabitol, and glycerol) as by-products. It is also accompanied with some medium components and components derived from the microorganisms. Therefore, it is difficult to obtain xylitol having a high purity when the culture broth of a xylitol-producing microorganism (a xylitol enzymatic reaction broth or a xylitol fermentation broth) is, as it is, concentrated to crystallize the xylitol. Furthermore, the aforementioned Published Japanese Translation No. 505522/1996 refers indeed to purification of xylitol obtained by fermentation, but there are disclosed no concrete procedures for purifying the xylitol. And, other than the publication, any procedures for purification of xylitol are not known for methods of producing xylitol, starting with glucose with the use of a microorganism. For these reasons, if highly pure xylitol can be recovered from a xylitol-containing solution produced by using a microorganism (a xylitol fermentation broth or a xylitol enzymatic reaction broth), it means that xylitol can be easily obtained in a commercially inexpensive way.
An object of the present invention is to provide a means for obtaining highly pure xylitol in a commercially effective way from a xylitol solution (a culture broth of a xylitol-producing microorganism, e.g., a xylitol enzymatic reaction broth, or a xylitol fermentation broth), in the method for producing xylitol by using a xylitol-producing microorganism.
As a result of extensive studies on production method of highly pure xylitol, the present inventors have found that highly pure xylitol can be obtained from a culture broth of a xylitol-producing microorganism such as a xylitol enzymatic reaction broth or a xylitol fermentation broth, by utilizing ion exchange resins ingeniously, and have accomplished the present invention based on such findings.
Accordingly, the present invention relates to a process for producing xylitol of high purity which comprises the steps of (1) removing the solid matter from a culture froth obtained by culturing a xylitol-producing microorganism in an aqueous culture medium, (2) desalting the resulting solid matter-removed culture broth by removing the ionic substances therefrom by means of a cation-exchange resin and an anion-exchange resin, (3) subjecting the resulting desalted solution to the chromatography using a strongly acidic cation-exchange resin to separate the xylitol from the other sugar alcohol(s) and sugar(s), and (4) obtaining the xylitol by separating it at a high purity from the resulting xylitol solution (fraction), and to a similar process but wherein the desalting carried out twice by adding the ion-exclusion step between Steps (1) and (2) of the above-mentioned process, i.e., a process for producing xylitol of high purity which comprises the steps of (1) removing the solid matter from a culture broth obtained by culturing a xylitol-producing microorganism in an aqueous culture medium, (2a) desalting the resulting solid matter-removed culture broth by removing most of the ionic substances therefrom by means of the ion-exclusion chromatography using a strongly acidic cation-exchange resin, (2b) desalting the solution resulting from the ion-exclusion chromatography (partially desalted solution) by removing the remaining ionic substances therefrom by means of a cation-exchange resin and an anion-exchange resin, (3) subjecting the resulting desalted solution to the chromatography using a strongly acidic cation-exchange resin to separate the xylitol from the other sugar alcohol(s) and sugar(s), and (4) obtaining the xylitol by separating it at a high purity from the resulting xylitol solution (fraction).